Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (1): 219-228.doi: 10.11743/ogg20220118

• Methods and Technologies • Previous Articles     Next Articles

Probability-constrained identification of Ordovician small-scale fractured-vuggy reservoirs in Blocks 4-6, Tahe Oilfield, Tarim Basin

Juan Zhang1,2(), Min Yang2, Runcheng Xie3, Ming Wang2, Hong Wang2, Ziwei Luo3   

  1. 1.School of Energy Resources, China University of Geosciences, Beijing 100083, China
    2.Northwest Oilfield Company, PetroChina, Urumqi, Xinjiang 830011, China
    3.School of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, China
  • Received:2021-07-06 Revised:2021-11-30 Online:2022-02-01 Published:2022-01-28

Abstract:

Various types of small-scale fractured-vuggy reservoirs occur in the Middle-Lower Ordovician of Blocks 4-6 in the Tahe Oilfield, Tarim Basin, and have been considered as the most potential targets for future oilfield development in the area. However, these reservoirs are difficult to identify due to their complex storage space that induces confusing responses during conventional well logging. To tackle the problem, this study starts with classifying the reservoirs according to drilling and logging data as well as core observation, continues with identifying logging responses typical of the reservoirs based on crossplotting well logging parameters, setting up a discriminant function by using the multivariate stepwise discriminant principle and ends with establishing a method for identifying the reservoirs under probability constraints by determining the parent posterior probability identification limits of all types of reservoirs. The results shows that the reservoirs can be classified into five types: unfilled vug, vug filled with sand, clay and gravel, fracture-vug, fracture-pore and dissolved cave-pore types, which can be recognized respectively with critical probability identification limits of 0.98, 0.98, 0.60, 0.60 and 0.60. These reservoirs are senstive to natural gamma, resistivity and density logging rays. The study suggests that reservoirs of fracture-vug, fracture-pore and cavern types are highly developed in the study area, but their development show great variation vertically in a single well and laterally in the study area. Block 4 develops more fractured-vug reservoirs than Block 6 and shows a vertical two-phase karstification. This identification method embodies well the idea of “step by step identification” and avoids the pitfall of confusing logging signals of these fractured-vuggy reservoirs. Using the method can effectively improve the identification of fractured-vuggy reservoirs, thus providing an effective way for a deeper understanding of the evolution and potential of the reservoirs.

Key words: conventional logging, probability recognition, small-scale fractured-vuggy reservoir, Ordovician, Tahe Oilfield, Tarim Basin

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